THE SKELETON 431 
and fuses with the trabecule, forming the septum interorbitale 
and septum nasi (Fig. 243). The free posterior border of this 
plate lies in front of the optic nerves; an interorbital aperture 
arises in the plate secondarily (Fig. 243). 
In the ethmoidal region the septum nasi arises as an anterior 
continuation of the interorbital plate; and the trabecular plate 
is continued forward as a prenasal cartilage in front of the olfac- 
tory sacs. Curved, or more or less rolled, plates of cartilage 
develop in the axis of the superior, middle, and inferior turbinals 
(see olfactory organ), and these are continuous with the lateral 
wall of the olfactory capsules, which in its turn arises from the 
dorsal border of the septum nasi (Figs. 248 and 244). 
(2) The Origin of the Visceral Chondrocranium (Cartilaginous 
Splanchnocranium). The visceral portion of the cartilaginous 
skull arises primarily in connection with the arches that bound 
the cephalic portion of the alimentary tract, viz., oral cavity 
and pharynx. In the chick, cartilaginous bars are formed in 
the mandibular arch, hyoid arch, and third visceral arch. In 
fishes, the posterior visceral arches also have an axial skeleton, 
but in the chick the mesenchyme of these arches does not develop 
to the stage of cartilage formation. The elements of these arches 
are primarily quite distinct. The upper ends of the mandibular 
and hyoid skeletal arches are attached to the skull; and the lower 
ends of the three arches concerned meet in the middle line. Two 
medial elements or copule are formed in the floor of the throat, 
one behind the angle of the hyoid arch, and one behind the 
third visceral arch (Fig. 245). 
Mandibular Arch. Two skeletal elements arise in the man- 
dibular arch on each side, a proximal one (the palato-quad- 
rate) and a distal one (Meckel’s cartilage). The former is 
relatively compressed, and the latter an elongated element (Fig. 
243, 10). The palato-quadrate lies external to the antero-ver- 
tral part of the auditory capsule, and soon develops a triradiate 
form. The processes are: the processus oticus, which applies 
itself to the auditory capsule, the processus articularis, which 
furnishes the articulation for the lower jaw, and the processus 
orbitalis, which is directed anteromedially towards the orbit. 
A small nodule of cartilage of unknown significance lies above 
the junction of the processus oticus and otic labyrinth. Meckel’s 
cartilage is the primary skeleton of the lower jaw, corresponding 
